Acquisitions - Purchase Price Allocation (Details) - USD ($) $ in Millions				9 Months Ended
	Mar. 01, 2017	Feb. 01, 2017	Dec. 23, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Business Acquisition [Line Items]
Purchase price, cash consideration				$ 184	$ 18
Business Combination, Goodwill				$ 138		$ 90
USM - Warren [Member]
Business Acquisition [Line Items]
Total purchase consideration	$ 78
Business Combination, Accounts receivable - Trade	17
Business Combination, Inventories	9
Business Combination, Other current assets	7
Business Combination, Goodwill	5
Business Combination, Intangibles	33
Business Combination, Property, plant and equipment	46
Business Combination, Accounts payable	(35)
Business Combination, Accrued payroll and employee benefits	(3)
Business Combination, Other accrued liabilities	(1)
Business Combination, Total purchase consideration allocation	$ 78
BFP and BPT [Member]
Business Acquisition [Line Items]
Purchase price, cash consideration		$ 181
Total purchase consideration		181
Business Combination, Cash and cash equivalents		75
Business Combination, Accounts receivable - Trade		78
Business Combination, Accounts receivable - Other		14
Business Combination, Inventories		137
Business Combination, Other current assets		6
Business Combination, Goodwill		29
Business Combination, Intangibles		41
Business Combination, Deferred tax assets		3
Business Combination, Other noncurrent assets		4
Business Combination, Property, plant and equipment		145
Business Combination, Notes payable, including current portion of long-term debt		(131)
Business Combination, Accounts payable		(51)
Business Combination, Accrued payroll and employee benefits		(14)
Business Combination, Other accrued liabilities		(18)
Business Combination, Long-term debt		(51)
Business Combination, Pension and postretirement obligations		(12)
Business Combination, Other noncurrent liabilities		(16)
Business Combination, Redeemable noncontrolling interest		(44)
Business Combination, Noncontrolling interests		(14)
Business Combination, Total purchase consideration allocation		$ 181
SIFCO SJT [Member]
Business Acquisition [Line Items]
Purchase price, cash consideration			$ 60
Purchase price, deferred consideration			10
Total purchase consideration			70
Business Combination, Accounts receivable - Trade			1
Business Combination, Accounts receivable - Other			1
Business Combination, Inventories			10
Business Combination, Goodwill			7
Business Combination, Intangibles			3
Business Combination, Property, plant and equipment			59
Business Combination, Accounts payable			(2)
Business Combination, Accrued payroll and employee benefits			(9)
Business Combination, Total purchase consideration allocation			$ 70